1. Field of the Invention
The invention concerns a fitting for sealed termination of a length of corrugated tubing, especially corrugated stainless steel gas lines, of the type that includes a generally cylindrical fitting body and nut, threaded together on opposite sides of a tubing retainer. The retainer has one or more inward rings or ridges, complementary with the tubing corrugations. The retainer grips the corrugated tubing near a cut end. Tightening the nut on the fitting body forces the retainer to advance the cut end of the tube axially into the fitting body.
According to an inventive aspect, the cut end is carried by the retainer against an axially facing surface in the fitting body surrounded by an annular groove. The annular groove is sized to receive the end of the retainer, with a slight annular clearance approximating the thickness of the tubing material. The endmost corrugation ridge is folded over into the annular groove, flattened between the retainer and a cylindrical side wall of the annular groove, and advanced axially with the end of the retainer into the annular groove. The result is a mechanical connection and a gas-tight seal between the tubing and the fitting body, that does not rely on accuracy of axial position or clamping pressure.
2. Prior Art
Flexible corrugated tubing, especially corrugated stainless steel with optional plastic cladding, is advantageous for natural gas supply lines and other connections that need to be gas-tight and/or water-tight as well as durably protected. The flexibility of such corrugated tubing allows variations in orientation and spacing of the tubing between opposite ends. The tubing is durable and resistant to damage from punctures and crushing. The flexibility minimizes metal fatigue cracking due to repeated flexing. By engaging in the corrugations, terminating fittings can make strong mechanical connections with the tubing, to bear substantial tension without being pulled apart or allowing leakage.
It is desirable to provide strong and reliable couplings. The corrugated tubing must be sealed and mechanically attached to associated terminal fittings to provide a leak resistant flow path. A terminal fitting for a tubing end could be or could engage with any of various structures, such as a rigid supply pipe having a pipe thread fitting, part of an appliance, an intermediate device such as a tee or a diameter changing nipple, a valve, manifold, filter, nozzle or burner, etc.
The mechanical connection and the seal between the corrugated tubing and the terminal device or fitting should remain hermetically tight and mechanically load bearing over the life of the connection. Often that time equates with the life of the associated appliance. The tubing may be used to carry flammable gas to an appliance, and should survive adverse conditions without leakage. For example, the seal should remain gas-tight even in high temperature conditions as one might expect in a fire.
Various terminal fittings for corrugated tubing are known and are intended to provide good mechanical connections and hermetic seals. Known fittings have a range of structures and complexity. Some aspects that distinguish fitting structures over one another, in addition to mechanical attachment and sealing effectiveness, include the expense, the number and complexity of the parts, the steps required to assemble the fitting on a tube end, whether the parts are consumed or re-usable, etc.
Establishing a seal typically involves clamping the tubing with axial and/or radial pressure. The clamping pressure is achieved by tightening together threaded parts of the fitting, normally by applying torque between a nut and a fitting body. Such pressure may be achieved in various ways by which axial approach of the threaded nut and fitting body results in a clamping force applied usefully to the corrugated tubing material. Axial force may be applied to force a corrugation ridge into position, or perhaps to pinch a corrugation ridge between axially approaching structures, or both.
For holding the corrugated tubing against axial displacement, retaining structures such as circumferentially split retainer rings or other cinching structures may engage with the tubing. A split retainer ring can have one or more annular ridges extending radially inwardly into the valleys between adjacent corrugation ridges of the tubing. The retaining structure thereby is fixed against axial displacement along the corrugations.
Two or more parts are brought axially together on gripping structure and the tubing gripped therein, especially for pushing an end of the corrugated tubing axially against one of the two parts. In some arrangements, conical surfaces also are employed. For example, advancing a split ring retainer axially into a narrowing conical structure can cinch the split ring retainer inwardly onto the tubing.
Preferably, simple assembly of a few parts is all that is required to assemble and seal the fitting. The assembly advantageously should be limited to placing a nut and retainer on the tubing and threading the nut onto the fitting body. Tightening the threaded parts presses the cut end of the tubing against a gasket, or compresses, crimps or pinches one or more corrugation ridges at the cut end, and thus seals the walls of the corrugated tubing against the fitting parts.
In a seal in which parts are threaded together to clamp down onto the tubing, there is a minimum tightness needed to achieve a seal, and there is a limit as to how far the parts can be tightened. Different assemblers may apply more or less torque to the fitting. It would be advantageous if he integrity of the seal did not depend on obtaining a specific degree of torque. It would also be advantageous if the assembler could confidently determine when sufficient torque had been applied, rather than feeling compelled to torque the fitting as much as possible.
Some fittings use a resilient axially-compressible gasket. A compressible gasket might seal sufficiently with a tubing end over a range of distances, due to the compression of the gasket. Also, a lack of precision in cutting the end of the tubing (such as defects in the smoothness of the cut edge, alignment of the cut of the plane normal to the axis of the tube, and the like) do not defeat sealing if the dimensional irregularity is less than or equal to the compression of the gasket.
Compressible gaskets also have drawbacks. Compressible materials for gaskets are generally less durable than metal, particularly the stainless steel of a corrugated tube. Compression can permanently compress and otherwise damage compressible gasket material, making the fitting unsuitable for re-use after detachment. Compressible materials harden with time. Compressible materials may be damaged by heat, combustion or exposure to chemicals. Compressible materials may complicate assembly because it is necessary to achieve a certain compression force, but it may damage a gasket to over-tighten and crush the gasket. The correct amount of compression is difficult for the assembler to assess by feel.
Sealing by contact between two metal surfaces does not turn on the extent of compression and in a metal-to-metal seal it may be advantageous to torque the fitting as heavily as possible. Metal/metal seals require some precision in the surfaces but provide durable connections that do not degrade in time, are less likely than a gasket to be corroded by contact with a transported medium, and are mechanically strong. Tolerances associated with the precision of metal surfaces can be important because metals are not readily compressible.
It is possible to envision an axially cut end of a corrugated tube being sealed by engagement against an axially facing planar surface in a fitting body. The cut end of the tubing would need to be smooth and placed exactly on a plane perpendicular to the axis of the tube. Different sorts of tools are used to make tubing cuts, which affect the nature of the cut (e.g., a hacksaw versus a pipe cutter). The cut edge may have burrs or irregularities. Some cutting techniques (e.g., a chop saw) can produce a cut edge at any phase position along the period of the corrugations, between the maximum and minimum diameter. These variations complicate the possibility of a direct endwise seal between the cut end and an abutting surface arranged substantially in a plane normal to the axis of the tube.
To reduce the possibility that unevenness at the cut end could result in a gap, some fittings clamp and flatten one or more corrugations of the tubing between vise-like abutting surfaces. The surfaces may be planar or conical but typically extend radially so as to be brought axially together toward abutment when tightening the fitting. These radial abutting surfaces flatten the endmost corrugation(s) into a flattened radial flange with flat annular sealing surfaces on opposite sides, in a plane normal to the longitudinal axis of the tubing. An example is disclosed by U.S. Pat. No. 4,630,850—Saka, wherein tubing corrugations are clamped between axially facing surfaces of a split ring retainer and an annular surface in a fitting body. Similar results are achieved if the clamping surfaces are conical, whether the conical surface is constricting or flaring with axial advance, as disclosed respectively in U.S. Pat. Nos. 6,173,995—Mau or 5,799,989—Albino. Although conical, these clamping surfaces operate in a manner similar to Saka, coming solidly together on one or more corrugations and being tightened as far as the assembler can manage.
It would be advantageous to maximize the benefits of metal to metal sealing strength by providing a coupling that forms a metal-to-metal seal that is not sensitive to the quality of the cut at the end of the tubing and which further does not rely upon the application of excessive force to flatten an end corrugation sufficiently to form a flat seal between annular sealing surfaces normal to the longitudinal axis of the tubing. At the same time, it would be advantageous to provide a fitting that achieves a sealing state that the assembler can sense as the fitting is tightened. Although not relying on extreme torque to seal, the fitting should also achieve a tightness whereby the fitting is difficult or impossible to unthread manually without the use of tools.